October 24, 2026
Valve seat leakage requirements describe how tightly a closed valve must shut off under defined test conditions. A specification should identify the test standard, acceptance limit, medium and pressure. Terms such as “zero leakage” or “bubble-tight” need that context. ANSI/FCI 70-2 Class VI, for example, permits a small, defined amount of seat leakage.
API 598 defines inspection and pressure tests, including leakage acceptance limits, for industrial valves. ANSI/FCI 70-2 defines control-valve seat leakage classes and their test procedures. ISO 5208 covers pressure testing and closure acceptance of metallic industrial valves using separate leakage-rate designations. This article explains their different scopes and how to specify the appropriate requirement. [1,2,3,4]
API 598, Valve Inspection and Testing, covers gate, globe, check, ball, butterfly and plug valves. It specifies required and optional tests, media, pressures, durations and acceptance limits. Use the edition required by the product standard or purchase documents; the API 598 discussion here refers to the 2023 edition.
Test selection depends on valve type, size, rating and construction. The table outlines common categories; not every test is required for every valve.
| API 598 test | Pressure basis | Purpose |
|---|---|---|
| Shell test | Table 2 for the valve material and rating | Checks the pressure boundary and body joints |
| High-pressure closure test | Table 3 for the applicable valve and seat rating | Checks closed-seat tightness against its acceptance limit |
| Low-pressure gas closure test | 5.5 ± 1.5 barg (80 ± 20 psig) | Checks seat tightness at the specified low gas pressure |
| Backseat test | Table 3 where the backseat test applies | Checks the backseat seal with the valve fully open |
API 598:2023 Table 5 provides closure-test leakage limits. Resilient-seated valves and metal-seated valves have different criteria, and metal-seated check valves have a separate allowance. Use the applicable category, size and test-medium column together with the table notes. A requirement for no visible seat leakage on a metal-seated valve may be an additional purchase requirement; it should not be assumed from “API 598” alone. [3]
Flanged ball valve with pneumatic actuator
ANSI/FCI 70-2-2021, Standard for Control Valve Seat Leakage Testing, addresses leakage past the closed seat of control valves. It is not a general classification of every isolation valve. Its seat-leakage provisions exclude control valves with a rated Cv below 0.1. ISO 5208 uses a separate system of lettered leakage rates, including intermediate designations; those rates are not direct equivalents of FCI Classes I–VI. [1,4]
FCI 70-2 defines six leakage classes. The table summarises their acceptance bases and corresponding test procedures. Typical design associations help explain common applications, but do not restrict which seat materials may achieve a class. [1,5]
Pneumatic single seat control valve
| Class | Acceptance basis | Test | Common association |
|---|---|---|---|
| I | No seat-leakage test required by agreement for a Class II, III or IV design | None | Same design intent as the underlying class |
| II | Up to 0.5% of rated valve capacity under the stated conditions | A | Double-seat or balanced metal-seat control valves |
| III | Up to 0.1% of rated valve capacity under the stated conditions | A | Tighter seat and seal performance than Class II |
| IV | Up to 0.01% of rated valve capacity under the stated conditions | A | Common metal-seat control-valve performance |
| V | Water: 0.0005 mL/min per inch of seat diameter per psi differential. Gas alternative: separate limit. | B or B1 | Tight metal-seat control-valve designs |
| VI | Small gas-leakage allowance tabulated by nominal seat diameter | C | Often resilient seats; selected metal-seat designs also qualify |
Class VI is commonly associated with resilient seating. Certain metal-seated control-valve designs can also meet Class VI requirements. The specified performance must be confirmed for the selected seat, trim and actuator configuration using the corresponding test procedure. [6]
Class V specifies a maximum allowable leakage under defined test conditions. A lower measured leakage, including no leakage detected by the test method, also meets the criterion. The class does not place an upper limit on the shutoff performance achievable with a metal seat, or predict the same leakage rate in service.
For the Class V Type B water test, the allowance is 0.0005 mL/min per inch of seat diameter per psi of differential pressure. For a valve with a 4-inch seat and a specified test differential of 100 psi, the calculated limit is 0.20 mL/min. Class V also includes the Type B1 low-pressure gas alternative, which has a separate acceptance limit. The purchase specification should identify the required procedure. [1,5,7]
Class VI uses the Type C procedure with air or nitrogen at 10–51°C (50–125°F). The test pressure is the lower of 3.5 barg (50 psig) and the valve’s maximum rated differential pressure. The leakage allowance is selected from the nominal seat-diameter table. Seat diameter must be confirmed separately from the valve body NPS or DN. A 2-inch seat, for example, has an allowance of 0.45 mL/min. [1,5]
For floating ball valves, wedge gate valves and resilient-seated butterfly valves used in isolation service, specify closure-test acceptance under the applicable product standard and purchase specification. If an FCI leakage class is required as an additional criterion, identify its test procedure and confirm performance for the selected configuration.
Resilient-seated flanged butterfly valve
Class VI permits a small, measurable gas leakage, so “bubble-tight” should be understood in relation to the specified test. Bubble counting is an alternative measurement method when the apparatus correctly represents the required volumetric leakage rate. Operating temperature, media compatibility, closing load and seat wear must also be considered when assessing shutoff performance in service. [2,5,6]
Dingliu supplies ball, butterfly and gate valves for water, HVAC, chemical and general industrial service. Send the required product standard, test standard and closure acceptance criterion with your inquiry so we can review the proposed configuration.
Select shutoff performance for the valve’s function and the consequence of leakage. FCI classes use different media, pressures and calculation bases, so compare the complete test requirements when choosing a class. High-temperature and abrasive duties also require a suitable seat and trim design for the actual operating conditions.
For general isolation duty, follow the specified product standard and its pressure-test requirements. For control duty, select an appropriate FCI class and procedure. Toxic, flammable or fuel-gas service requires a service-specific specification; Class V or VI alone does not establish safe isolation, fire resistance or external-emissions performance. State any additional tests and operating shutoff requirements separately.
The following matrix summarises the specification basis and information needed for common duties.
| Valve duty | Specification basis | What to confirm |
|---|---|---|
| General isolation | Applicable product standard plus API 598 or another specified pressure-test standard | Seat category, direction, pressure and closure acceptance limit |
| Modulating control | Specified FCI 70-2 class and its corresponding procedure | Rated capacity or seat diameter, actuator setup and test conditions |
| Toxic, flammable or fuel-gas service | Service-specific isolation or control requirements | Required shutoff performance and any separate fire or emissions tests |
| High temperature or abrasive media | Service-compatible trim and an agreed acceptance criterion | Actual temperature, pressure, wear conditions and maintenance needs |
For a control valve, an RFQ may state: “Seat leakage to ANSI/FCI 70-2-2021 Class VI, Type C, using air; test pressure to clause 5.4.2. Report seat diameter, test direction, closing load and measured leakage.” Specify shell testing under the applicable product or inspection standard separately. For an isolation valve, state the applicable API 598 closure acceptance category, or the required ISO 5208 leakage rate, rather than combining their procedures with an FCI label.
No. Class VI belongs to ANSI/FCI 70-2, not API 598. If both standards are required, identify the tests governed by each. For example, specify control-valve seat testing to FCI 70-2 Class VI, Type C, and specify shell testing separately under the applicable product standard. An API 598 seat-test result cannot automatically be relabelled as an FCI Class VI result.
The limit depends on the seat category, valve size and test medium. API 598 includes closure acceptance limits as well as methods and durations. Check the applicable edition’s leakage table and notes, and any additional purchase requirements. Metal-seated gate valves must not be assessed using the separate allowance for metal-seated check valves.
The closed valve is tested with air or nitrogen under the Type C conditions. Leakage is measured with suitable equipment and compared with the seat-diameter allowance. For a 4-inch seat, the table limit is 1.70 mL/min. Bubble counts depend on the measuring apparatus, so a generic bubbles-per-minute value is not a substitute for the specified flow limit. [1,5]
Class VI is appropriate when its defined seat-leakage performance meets the control-valve application. It is not restricted to soft seats, and it is not automatically the correct requirement for an isolation valve. Confirm performance for the selected seat, trim, actuator and service conditions. A different acceptance standard may be more suitable for the specified isolation duty.
FCI 70-2 provides control-valve seat leakage classes and production test procedures. ISO 5208 covers pressure-boundary integrity and closure tightness of metallic industrial valves, with lettered leakage rates. ISO 5208 Rate A means no visually detectable leakage during the specified test; FCI Class VI permits a small tabulated gas leakage. A numerical resemblance between other limits does not establish equivalence of the complete tests. [1,4,7]
A factory test demonstrates performance under its defined conditions. Operating temperature, media, differential pressure, wear, deposits and actuator settings can change shutoff behaviour. If leakage develops in service, review these factors alongside the original test record and operating history to identify the cause.
API 598, ISO 5208 and FCI 70-2 each include acceptance requirements within their own scope. Select the appropriate document for the valve and duty, state its edition and acceptance criterion, and retain the relevant test conditions. Seat leakage is internal flow past the closed valve; external fugitive emissions require a separate assessment. A clear specification links the required shutoff performance to the actual seat design, actuator and operating conditions.
Send the medium, operating temperature, pressure, maximum shutoff differential and required leakage criterion. We will review the valve and seat selection together with the requested inspection and testing requirements.
[1] ANSI/FCI 70-2-2021 — Standard for Control Valve Seat Leakage Testing. FCI standard listing.
[2] Fluid Controls Institute — ANSI/FCI 70-2 informational sheet.
[3] API 598:2023 — Valve Inspection and Testing. Tables and requirements must be read with their notes.
[4] ISO 5208:2015 — Industrial valves — Pressure testing of metallic valves.
[5] Emerson Fisher — Catalog 12, Section 2, seat leakage technical information.
[6] Emerson Fisher — easy-e Cryogenic Valves, D102189X012. Examples of model-specific metal-seat shutoff options.
[7] Valmet — North America seat leakage guide, T142-1EN, October 2022. Supplementary calculation examples; use the edition specified for the purchase.
|
Ready to Specify Your Valves? Send us your application — media, temperature, pressure and target leakage class — and the Dingliu engineering team will confirm the right valve and seat design, tested per API 598. |
Tianjin Dingliu valve Industry Co., Ltd. is a high-tech enterprise specializing in the valve and automation industry.
Privacy Policy | SiteMap Copyright Tianjin Dingliu valve Industry Co., Ltd.
